A fuel cell comprises a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte interposed between the electrodes and means for separately supplying a stream of fuel and a stream of oxidizer to the anode and the cathode, respectively. In operation, fuel supplied to the anode is oxidized releasing electrons which are conducted via an external circuit to the cathode. At the cathode the supplied electrons are consumed when the oxidizer is reduced. Proton exchange membrane fuel cells use a solid proton-conducting polymer membrane as the electrolyte.
Conventional fuel cells use hydrogen gas as the fuel. Pure hydrogen gas, however, is difficult and costly to supply. Thus, hydrogen gas is typically supplied to a fuel cell using a reformer, which steam-reforms methanol and water to a hydrogen-rich fuel gas containing carbon dioxide. Theoretically, this “reformate” gas consists of 75 vol. % hydrogen and 25 vol. % carbon dioxide. In practice, however, this gas also contains nitrogen, oxygen and, depending on the degree of purity, varying amounts of carbon monoxide. This process is complex, and the conversion of a liquid fuel directly into electricity would be desirable, as then a high storage density, system simplicity and retention of existing fueling infrastructure could be combined. Methanol is an especially desirable fuel because it has a high energy density, low cost and is produced from renewable resources. Thus, there is now a strong interest in the direct methanol fuel cell, in which the overall process that occurs is methanol and oxygen react to form water and carbon dioxide and electricity.
Conventionally platinum has been used as the cathode catalyst for ORR in PEM fuel cells. Because of the high cost of platinum, there has been interest to find non-platinum catalysts which will have comparable or increased activity relative to platinum. Palladium has been proposed as an alternative to platinum, because palladium is available at lower cost. Palladium-cobalt binary alloys have shown useful activity for ORR. However the present inventors have found that the stability of such alloys is less than is desired in practical applications for PEM fuel cells.
It is an object of this invention to provide palladium-based ternary alloys which are effective catalysts for ORR and which have improved stability relative to palladium-based binary alloys.